Subject: Re: [v3,11/41] mips: reuse asm-generic/barrier.h

From: "Paul E. McKenney" <[email-masked]>
Date: Tue, 26 Jan 2016 15:37:33 -0800

On Tue, Jan 26, 2016 at 12:10:10PM +0000, Will Deacon wrote:
> On Mon, Jan 25, 2016 at 05:06:46PM -0800, Paul E. McKenney wrote:
> > On Mon, Jan 25, 2016 at 02:41:34PM +0000, Will Deacon wrote:
> > > On Fri, Jan 15, 2016 at 11:28:45AM -0800, Paul E. McKenney wrote:
> > > > On Fri, Jan 15, 2016 at 09:54:01AM -0800, Paul E. McKenney wrote:
> > > > > On Fri, Jan 15, 2016 at 10:24:32AM +0000, Will Deacon wrote:
> > > > > > See my earlier reply [1] (but also, your WRC Linux example looks more
> > > > > > like a variant on WWC and I couldn't really follow it).
> > > > > 
> > > > > I will revisit my WRC Linux example.  And yes, creating litmus tests
> > > > > that use non-fake dependencies is still a bit of an undertaking.  :-/
> > > > > I am sure that it will seem more natural with time and experience...
> > > > 
> > > > Hmmm...  You are quite right, I did do WWC.  I need to change cpu2()'s
> > > > last access from a store to a load to get WRC.  Plus the levels of
> > > > indirection definitely didn't match up, did they?
> > > 
> > > Nope, it was pretty baffling!
> > 
> > "It is a service that I provide."  ;-)
> > 
> > > > 	struct foo {
> > > > 		struct foo *next;
> > > > 	};
> > > > 	struct foo a;
> > > > 	struct foo b;
> > > > 	struct foo c = { &a };
> > > > 	struct foo d = { &b };
> > > > 	struct foo x = { &c };
> > > > 	struct foo y = { &d };
> > > > 	struct foo *r1, *r2, *r3;
> > > > 
> > > > 	void cpu0(void)
> > > > 	{
> > > > 		WRITE_ONCE(x.next, &y);
> > > > 	}
> > > > 
> > > > 	void cpu1(void)
> > > > 	{
> > > > 		r1 = lockless_dereference(x.next);
> > > > 		WRITE_ONCE(r1->next, &x);
> > > > 	}
> > > > 
> > > > 	void cpu2(void)
> > > > 	{
> > > > 		r2 = lockless_dereference(y.next);
> > > > 		r3 = READ_ONCE(r2->next);
> > > > 	}
> > > > 
> > > > In this case, it is legal to end the run with:
> > > > 
> > > > 	r1 == &y && r2 == &x && r3 == &c
> > > > 
> > > > Please see below for a ppcmem litmus test.
> > > > 
> > > > So, did I get it right this time?  ;-)
> > > 
> > > The code above looks correct to me (in that it matches WRC+addrs),
> > > but your litmus test:
> > > 
> > > > PPC WRCnf+addrs
> > > > ""
> > > > {
> > > > 0:r2=x; 0:r3=y;
> > > > 1:r2=x; 1:r3=y;
> > > > 2:r2=x; 2:r3=y;
> > > > c=a; d=b; x=c; y=d;
> > > > }
> > > >  P0           | P1            | P2            ;
> > > >  stw r3,0(r2) | lwz r8,0(r2)  | lwz r8,0(r3)  ;
> > > >               | stw r2,0(r3)  | lwz r9,0(r8)  ;
> > > > exists
> > > > (1:r8=y /\ 2:r8=x /\ 2:r9=c)
> > > 
> > > Seems to be missing the address dependency on P1.
> > 
> > You are quite correct!  How about the following?
> 
> I think that's it!
> 
> > As before, both herd and ppcmem say that the cycle is allowed, as
> > expected, given non-transitive ordering.  To prohibit the cycle, P1
> > needs a suitable memory-barrier instruction.
> > 
> > ------------------------------------------------------------------------
> > 
> > PPC WRCnf+addrs
> > ""
> > {
> > 0:r2=x; 0:r3=y;
> > 1:r2=x; 1:r3=y;
> > 2:r2=x; 2:r3=y;
> > c=a; d=b; x=c; y=d;
> > }
> >  P0           | P1            | P2            ;
> >  stw r3,0(r2) | lwz r8,0(r2)  | lwz r8,0(r3)  ;
> >               | stw r2,0(r8)  | lwz r9,0(r8)  ;
> > exists
> > (1:r8=y /\ 2:r8=x /\ 2:r9=c)
> 
> Agreed.

OK, thank you!  Would you agree that it would be good to replace the
current xor-based fake-dependency litmus tests with tests having real
dependencies?

							Thanx, Paul